Mac Mini is 1.5 years old, Mac Pro is >2 years old, 13" Macbook Pro is 1 year next month (and conspicuously absent from rumors of Apple's upcoming announcement event, gotta release those watch bands though).

The iPhone is definitely on a 1-year cycle, but it's not really something they put effort into maintaining on their computers. MacBook Air and iMac are probably the closest.

I blame iOS/Mac features tie-in like Continuity. OS X definitely gets the short end of the stick from that.